(1) Field of the Invention
This invention relates to a liquid composition useful as a heat-absorbing medium in water heaters and heat collectors designed for harnessing solar heat.
(2) Description of the Prior Art
The idea of exposing a black liquid to the sunlight thereby causing the liquid to absorb solar heat, then allowing the heated liquid to elevate the temperature of ordinary tap water by exchange of heat, and utilizing the resultant hot water for household or industrial uses has been known in the art. As the liquid for the absorption of solar heat, a solution of a black dye in water and a dispersion of a black pigment in water are used, for example. Heat-absorbing liquids using dyes are not feasible because they fail to withstand prolonged exposure to the sunlight. On the other hand, heat-absorbing liquids using black pigments such as carbon black show high stability under the sunlight. When a heat-absorbing liquid obtained by dispersing carbon black in water with the aid of a dispersant is circulated through pipes in a water heater or a heat collector and caused to absorb solar heat and then passed through a heat exchanger to elevate the temperature of tap water, the carbon black particles dispersed in this liquid are separated and deposited on the inner wall of the pipes and the pipes are divested of capacity for thermal absorption. Further, because of the repeated fluctuation of temperature and owing to the action of the sunlight exerted over a long period, the dispersion has its thermal stability gradually lowered, with the result that carbon black particles will sediment possibly to an extent of depriving completely the heat-absorbing liquid of its function.